CLN: let Index use general concat machinery - remove Index._concat_same_dtype

[jorisvandenbossche]

Following up on the recent changes in the concat_compat machinery (to use the _get_common_dtype protocol for EAs), we can also use this in the Index class.

Doing this also makes the _concat_same_dtype method unnecessary (as this is handled in concat_compat to use EA._concat_same_type when appropriate)

cc @jbrockmendel

[jbrockmendel]

nice cleanup

[jreback]

thanks!